Optical inspection of machined surfaces

Abstract

Defects in a machined surface are detected by sensing excursions in a test signal generated by means of a conventional scanning system (1), which includes a detector responsive to light reflected and/or scattered from an illuminated spot scanned over the surface. The test signal is applied to a comparator (6), together with a signal representative of an appropriate threshold level for sensing the excursions. This level is ascertained, in accordance with a specific formula, by means of an analysis system (5) to which is applied a preliminary signal derived in a similar manner to the test signal from the surface to be inspected.

I claim: 
     
       1. In a method of inspection of a machined surface, which method comprises deriving an electrical test signal in respect of said surface by causing an illuminated spot formed by a beam of light to scan over said surface and detecting the resultant variations of the proportion of the incident light returned from said spot, and sensing excursions in said test signal corresponding to decreases of said proportion below a preset threshold level: the improvement comprising setting said threshold level in accordance with the result of an analysis of an electrical signal derived in respect to said surface in a similar manner to that in which said test signal is derived, said threshold level being set substantially to the value given by the expression ##EQU3##  N A  and N B  being different values of N, there N is the fraction of the total length of the scan for which the value of said proportion falls below a given level L, and L A  and L B  being the values of L which respectively correspond to N A  and N B .   
     
     
       2. A method according to claim 1, in which the same beam of light is used in the derivation of both said test signal and the signal which is analysed. 
     
     
       3. A method according to claim 2, in which the signal which is analysed and said test signal are respectively derived by means of two consecutive scans of said surface which respectively involve movements of said spot in opposite senses along the same path on said surface. 
     
     
       4. A method according to claim 1, in which said surface is the surface of a cylinder bore, and said spot is caused to scan over said surface along a helical path having a pitch less than the diameter of said spot. 
     
     
       5. An apparatus for use in the inspection of machined surfaces, the apparatus comprising: scanning means for generating electrical signals in respect of a surface to be inspected by causing an illuminated spot formed by a beam of light to scan over said surface and detecting the resultant variations of the proportion of the incident light returned from said spot;   means for analysing a first signal generated by said scanning means in respect of a given surface to obtain data from which may be deduced the value of the expression ##EQU4##  N A  and N B  being different values of N, where N is the fraction of the total length of the scan for which the value of said proportion falls below a given level L, and L A  and L B  being the values of L which respectively correspond to N A  and N B  ;   means for sensing excursions in a second signal generated by said scanning means in respect of said given surface, said excursions corresponding to decreases of said proportion below a threshold level; and   means for setting said threshold level substantially to the value of said expression indicated by said data.